Spark plug seal



Nov. 29, 1960 A. CANDELISE 2,962,543

SPARK PLUG SEAL Filed July 25, 1957 1N VEN TOR.

a /fw ATTORNY United States Patent 2,962,543 SPARK PLUG SEAL Flint, Mich., assignor to General Motors Detroit, Mich., a corporation of Dela- Alfred Candelise, Corporation, ware This invention relates to an improved spark plug and more particularly to a spark plug having a long tip insulator with short tip or cold running operating characteristics.

Pie-ignition of engine fuel has always been a problem in the design of ignition systems and components thereof for use in internal combustion engines. The problem has been attacked from the standpoint of spark plug design by providing plugs having cold running characteristics, that is, relatively low insulator firing tip temperatures. This has been accomplished by designing the insulator with a relatively short firing tip extending below the lowermost insulator shoulder which is designed to be in good heat transmitting contact with the sealing ledge of the shell upon which it is normally positioned. In this manner, the heat path through the relatively poor heat conducting insulator is relatively short, the remaining heat path being through the metal shell which has ex cellent heat conducting characteristics.

While the designconstruction describedabove has been satisfactory from the standpoint of low operating temperatures of the spark plug, serious problems have resulted from this design. Specifically, the lower insulator shoulder and cooperating sealing ledge of the shell .arepositioned so as to be located adjacent-the threaded portion of the shell with the resultthat the application of excessive plug installation torque tends tostretch the shell between the sealing shoulders of the insulator with the result that the ,gastight seal of the plug is broken. Similarly, the breaking torque of the shell threaded portion is substantially lowered by reason of the pre-s'tressing of the shell when placed under tension in assembly of the plug with the lower shoulder of the insulator in gas-tight relationship with the cooperating sealingledge formed in the shell.

Accordingly, it is an object of my invention to provide a spark plug having .a substantially unstressed threaded shell portion while having cold running operating characteristics. It is another object of .my invention .to provide a spark plug having gas-tight sealing portions above the threaded portionuand having a short insulator firing tip. It is a further object .of my invention to provide an improved spark plug having an unstressed threaded shell portion While at t-hesame time provi' ag a short insulator firing tip having'good-heat' trans 'tting interconnection with the shell. 1 a

The improved spark plug of my invention is achieved by providing an insulator having upper and lower sealing shoulders positioned in the shell portion above the threaded lower end of the shell while providing means on the insulator between the lower sealing shoulder and the firing tip thereof to form an auxiliary seal between the insulator and the shell adjacent the firing tip.

These and other objects of my invention will be readily apparent from the specification which follows when read in connection with the drawing in which Figure 1 shows a cross-sectional view of a spark plug embodying the prin- 2 ciples of my invention; and Figure 2 shows the auxiliary sealing portion of the spark plug in enlarged view.

Referring now to the drawing, there is shown a spark plug 1 comprising an elongated tubular insulator member 3 provided with a center electrode 5 in the centerbore thereof thus constituting an insulator subassembly which is positioned in a generally tubular metal shell 7 having a ground electrode 9 secured in the lower end thereof. A metal shielding sleeve 11 is telescopically positioned within the upper end of the shell 7 and surrounds the upper portion of the insulator 3 in a manner more fully described hereinafter.

The insulator 3 is formed with an enlarged portion intermediate the ends thereof comprising a lower shoulder 13 and an upper shoulder 15. The upper portion 17 of the insulator 3 is adapted to receive the end of a shielded ignition cable (not shown) having means for establishing electrical contact with the center electrode 5.

As is clearly shown on the drawing, the insulator subassembly is positioned on an annular ledge 19 formed on the inner wall surface of the shell 7 above the threaded portion on the lower end of the outer wall surface of the shell, the portion of the insulator 3 extending below the shoulder 13 being spaced apart from and out of contact with the shell. A relatively soft high temperature resistant metal gasket 21 such as nickel or nickel alloy is positioned between the lower shoulder 13 and the ledge 19 to assure a gas-tight seal therebetween and to compensate for any surface unevenness on either the shoulder 13 or the ledge 19. The lower end of the shield sleeve 11 is positioned on the upper shoulder 15 of the insulator and is shaped to correspond with the surface contour of the shoulder. A soft metal gasket 23 such as copper or nickel is positioned between the shoulder 15 and the lower end of the sleeve 11 for the same purposes as stated with reference to the lower gasket 21.

As is also shown clearly in Figure l, the top of the shell 7 is turned or 'crimped inwardly so as to form an annular flange 25 pressed over onto an annular ledge 27 formed on the outer surface of the shield sleeve 11. In order to facilitate the crimping operation, the top of the shell is preferably of slightly reduced wall thickness. The mechanical interlock formed by means of the flange 25 and the ledge 27 precludes the disassembly of the shield sleeve 11 from the shell 7. Rotation between the parts is precluded by means of a braze or weld 29 placed upon the flange 25 and the sleeve 11.

In order to obtain a plug having cold running operating characteristics, I provide an auxiliary sealing shoulder 31 upon the lower end of the insulator between the firing tip thereof and the lower shoulder 13 and adjacent the threaded portion of the shell 7. Similarly, I provide an auxiliary sealing ledge 33 upon the inner surface of the shell adjacent the threaded portion thereof, the ledge 33 facing the upper end of the shell in order to form a seating surface for cooperation with the shoulder 31 formed on the insulator. As is clearly shown in Figure 2, in my preferred construction the maximum diameter of the auxiliary sealing shoulder 31 formed on the insulator 3 is less than the minimum diameterof the ledge 33 by a small amount shown on the drawing as D. Positioned between the ledge 33 and the shoulder 31 is a relatively soft heat-resistant gasket 35 such as nickel or nickel alloy for the purpose of providing a seal between the ledge and the shoulder. The difference in diameters represented in Figure 2 as D is a small but sufiicient amount to enable the shoulder 31 to bend the gasket 35 downwardly about the inner edge 37 of the ledge 33, the gasket thus acting as a cantilever member. In this manner, the hot combustion gases are precluded from contacting the portion of the insulator below the shoulder 13 and above the gasket 35. Good heat conducting contact is obtained between the insulator shoulder 31 and the ledge 33 with the result that the heat of the combustion gases is rapidly conducted away from the insulator with the temperature of the firing end of the insulator being kept at a relatively low temperature since the length of insulator exposed to combustion gases is kept at a desired minimum. Improved heat conductive contact between the insulator 3 and shell 7 may be obtained by forming the gasket 35 with an upwardly extending sleeve portion 39 which is adapted to be pressed onto the insulator body above shoulder 31 and have a press fit with the inner wall surface of the shell above ledge 33. As shown on the drawing. the resulting gasket is generally cup-shaped.

The inner edge 37 of the ledge 33 acts as a fulcrum or pivot point for the cantilever gasket 35 by reason of the difference D between the diameters of the edge 37 and the shoulder 31 of the insulator. It is obvious that upon positioning the insulator within the shell 7, the shoulder 31 will tend to bend the gasket 35 about the inner edge 37 of the shoulder 33 without placing the shell 7 under undue tension and without placing the insulator under compression. In this manner, critical manufacturing tolerances with respect to the distance between shoulders 13 and 31 and that between ledges 19 and 33 as well as critical thicknesses of gaskets 21 and 37 are eliminated. These distances and thicknesses may therefore be so designed when using the cantilever gasket of my invention as to obtain contact between the shoulder 31 and the cantilever gasket 35 before applying any sealing pressure to the primary sealing shoulders 13 and 15 of the insulator without danger of cracking the insulator or placing the threaded portion of the shell under any substantial strain. As an example of the auxiliary seal dimensions, I have obtained the desired results by forming an insulator having a shoulder 31 of 0.426 maximum diameter positioned on a nickel gasket 35 of 0.040" thickness which is seated on a ledge 33 having a minimum diameter of 0.450".

In assembling the spark plug in accordance with my invention, the insulator 3 having the center electrode assembled therewith is positioned within the shell 7, the gaskets 21 and 23 being positioned on the ledge 19 and shoulder 15, respectively. Cantilever gasket 35 is positioned on the shoulder 31 of the insulator 3 by being press fitted thereon. The shielding sleeve 11 is then positioned telescopically within the upper end of the shell 7 to bear upon the upper gasket 23, axial pressure being applied to the upper end of the sleeve to form a gas-tight seal between insulator shoulders 13 and 15 and ledge 19 of the shell and the lower end of the sleeve, respectively, the upper end of the shell 7 being crimped over onto the ledge 27 to form the interlocking flange 25. As noted above, a braze 29 is provided between the flange 25 and the sleeve 11.

During the assembly operation and prior to the application of axial sealing pressure upon the end of sleeve 11, contact is made between the cantilever gasket 35 and the ledge 33. Upon application of further pressure to the end of sleeve 11, the shoulder 31 bends the gasket 35 about the edge 37 of the ledge 33 to form a seal therebetween without placing any substantial strain upon either the insulator 3 or the shell 7.

With the parts thus assembled, the seal of the insulator 3 within the shell 7 may be further assured by Cico welding, that is, by applying high current through the shell to heat and soften an annular thin wall section 41 thereof while applying pressure to the ends of the shell to collapse section 41 and press insulator shoulder 13 into tighter contact with the gasket 21 positioned on ledge 19. It should be noted that such Cico welding operation places no further strain upon the insulator 3 and the shell 7 in the area of the cantilever gasket 35 for the reasons described above.

It is apparent from the foregoing description that I have provided a simple and effective means for achieving cold running operating characteristics in a spark plug even though the insulator tip below the primary sealing shoulders is relatively long. While I have described my invention in terms of a shielded spark plug, other embodiments of my invention will be readily apparent to those skilled in the art and such embodiments are within the intended scope of the claims which follow.

I claim:

1. In a spark plug, a generally tubular metal shell having a lower externally threaded portion, an upper sealing ledge above said threaded portion and a lower ledge on the interior surface of said threaded portion, a metal gasket seated on said upper sealing ledge, a generally cylindrical insulator subassembly having a portion of enlarged external diameter engaging said gasket to form a gas tight seal between said insulator and said shell, a lower shoulder formed on said insulator adjacent to said lower ledge, an annular metal gasket wedged betwen said shoulder and said lower ledge, a lower annular portion of said gasket being bent downwardly about the edge of said lower ledge, the maximum diameter of said shoulder being less than the minimum diameter of said lower ledge by a small but suflicient amount to enable said shoulder to bend said gasket Without substantial strain upon either the insulator or the shell adjacent said threaded portion.

2. In a spark plug comprising a shell having a threaded portion on the lower outer surface, a sealing ledge formed on the inner surface of said shell above said threaded portion, and an insulator subassembly comprising an insulator and center electrode positioned upon said ledge in gas-tight relationship therewith, the invention comprising a shoulder formed on said insulator between the lower end thereof and the portion positioned on said ledge, a cup-shaped gasket positioned upon said shoulder in good heat conducting relationship with said insulator, a second ledge on the inner wall surface of said shell correspondingly located with said shoulder, and said cup-shaped gasket being seated on said second ledge and being in good heat conducting relationship with said shell, the maximum diameter of said shoulder being less than the minimum diameter of said second ledge by a small but sufficient amount to enable said cup-shaped gasket to be bent about the edge of the ledge without placing any substantial strain on either the shell or the insulator.

References Cited in the file of this patent UNITED STATES PATENTS 1,607,308 Rohde Nov. 16, 1926 1,835,574 Rabezzana Dec. 8, 1931 2,245,153 McWane June 10, 1941 

